The present invention relates in general to the art of earth boring and, more particularly, to a down-the-hole fluid operated rock drill hammer. Fluid operated rock drill hammers generally include an annular body portion having a central chamber. A piston is mounted in the central chamber for axial movement to provide hammer blows. A bit is connected to the annular body for receiving the hammer blows. Fluid passage means are provided in the annular body and the piston for delivering driving fluid to move the piston and alternately strike the hammer blows and recover therefrom. In the prior art a phenomenon would occur known as "back hammering." Back hammering tended to occur in the prior art rock drill hammers when the hammer was off bottom because there was an absence of a positive force to maintain the piston in the lowermost position. Thus, when a hammer operation was to be discontinued, the drill pipe was raised allowing the bit to drop to its lowermost position. Under normal circumstances the piston should also drop to its lowermost position and remain there, however, because of inadequate design or because of fluid leakage due to wear, the piston would often tend to "float" above the bit. This would produce an unintended hammering action on the bit. This back hammering was very detrimental to the anvil and hammer surfaces and other elements of the rock drill hammer.